(1) Field of the Invention
The present invention relates to an optical attenuator utilizing the Faraday effect to thereby obtain variable optical characteristics, and particularly relates to a variable optical attenuator to achieve the reduction of a wavelength-dependency of the optical characteristics of the variable optical attenuator, and to an optical transmission system utilizing such a variable optical attenuator.
(2) Related Art
As components of an optical transmission system, for example, there have been widely used variable optical attenuators utilizing the Faraday effect which is a kind of magneto-optical effect. Variable optical attenuators and the like utilizing the Faraday effect are used to keep output levels of optical amplifiers constant, such as in a system having the optical amplifiers. Generally, such a variable optical attenuator is characterized in that the rotation angle of a Faraday rotator is varied by changing such as an electric current applied to an electromagnet for the Faraday rotator, and an optical attenuation amount of the variable optical attenuator is determined corresponding to the setting of the rotation angle, resulting in a constitution free of mechanical movable portions.
Concrete constitutions of conventional variable optical attenuators utilizing the Faraday effect include those described in Japanese Unexamined Patent Publication Nos. 61-35428 and 6-51255.
Meanwhile, Faraday rotators applied to the aforementioned conventional variable optical attenuators physically possess wavelength-dependencies corresponding to Faraday rotation angles. Further, it is also known that a certain degree of wavelength-dependency is generated in an optical attenuation amount, in a constitution where Faraday rotation angles are variably controlled by a synthesized magnetic field formed of mutually orthogonal fixed magnetic field and variable magnetic field, such as in the conventional variable optical attenuator described in the aforementioned Japanese Unexamined Patent Publication No. 6-51255. As such, in the conventional variable optical attenuator, levels of output lights differ from one another corresponding to the wavelengths of the lights, due to respective wavelength-dependencies of a Faraday rotation angle and of an optical attenuation characteristic. For example, constituting a wavelength-division multiplexing (WDM) optical transmission system by utilizing such a conventional variable optical attenuator results in mutually different light levels for every wavelength channels, thereby undesirably causing a nonlinear effect on a transmission path.
The present invention has been carried out in view of the conventional problems as described above, and it is therefore an object of the present invention to provide a variable optical attenuator to achieve the reduction of a wavelength-dependency of the entire device, by optimizing the magneto-optical system of the variable optical attenuator by deliberating the wavelength-dependency of Faraday rotation angles and to provide an optical transmission system utilizing such a variable optical attenuator.
To achieve the above object, one aspect of a variable optical attenuator utilizing the Faraday effect the present invention, comprises: a Faraday rotator for providing polarized light transmitted through the Faraday rotator with a variable Faraday rotation angle; and elements for extracting linearly polarized light arranged on the light axis in front of and behind the Faraday rotator, respectively, in which an optical attenuation amount of the variable optical attenuator is changed by a variation of the Faraday rotation angle, wherein an angle formed between the optical axis of one of the elements for extracting the linearly polarized light and the optical axis of the other of the elements is set such that the Faraday rotation angle at which the wavelength-dependency of the optical attenuation amount of the variable optical attenuator becomes the maximum, is brought to be substantially 0xc2x0.
According to such a constitution, the angle formed between the optical axis of one of the elements for extracting the linearly polarized light and the optical axis of the other of the elements is set such that the Faraday rotation angle, at which the wavelength-dependency of the optical attenuation amount of the variable optical attenuator becomes the maximum, is brought to be substantially 0xc2x0, in consideration of such a tendency that as the Faraday rotation angle becomes smaller, the wavelength-dependency is reduced, in view of the Faraday rotation angle at which the wavelength-dependency of the optical attenuation amount of the variable optical attenuator becomes the maximum. This results in a reduced wavelength-dependency of the optical attenuation amount at the Faraday rotation angle brought to be substantially 0xc2x0.
In the aforementioned variable optical attenuator, it is preferable that the variable optical attenuator further comprises magnetic field generating means for applying a fixed magnetic field and a variable magnetic field, directions of which are mutually different, to the Faraday rotator, wherein the Faraday rotation angle is settable at 0xc2x0, by setting the directions of the fixed magnetic field and the variable magnetic field so as to include such a state that the direction of a synthesized magnetic field cooperatively formed by the fixed magnetic field and the variable magnetic field becomes perpendicular to the light direction. Concretely, when the direction of the fixed magnetic field is parallel to the light direction, the direction of the variable magnetic field may form an obtuse angle relative to the direction of the fixed magnetic field. Alternatively, when the direction of the fixed magnetic field is not parallel to the light direction, the direction of the variable magnetic field may form an obtuse angle relative to the light-directional component of the fixed magnetic field.
According to such constitutions, the light-directional component of the magnetization of the Faraday rotator becomes zero under the condition that the direction of the synthesized magnetic field cooperatively formed by the fixed magnetic field and the variable magnetic field becomes perpendicular to the light direction, to thereby allow realization of the Faraday rotation angle of 0xc2x0.
Another aspect of a variable optical attenuator utilizing the Faraday effect according to the present invention, comprises: two polarizers for extracting polarized light, respectively; a Faraday rotator disposed between the polarizers; a first magnet for applying, a magnetic field thereof parallel to an axis of light passing through the Faraday rotator, to the Faraday rotator; and a second magnet having a magnetic field direction forming an obtuse angle relative to the magnetic field of the first magnet, wherein the strength of the magnetic field of the second magnet is adjustable. Further, in the aforementioned variable optical attenuator, the constitution may be such that the direction of a synthesized magnetic field to be cooperatively formed by the magnetic fields of the first magnet and the second magnet can be brought to become perpendicular to the axis of light, by adjusting the strength of the magnetic field of the second magnet.
Further objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments when read in conjunction with the accompanying drawings.